1. Field of the Invention
The present invention relates to cellular senescence, markers of cellular senescence, and methods and reagents for identifying senescent cells and agents that alter senescent gene expression expression of cells, as well as for detecting, diagnosing, preventing, and treating senescence-related diseases and conditions in humans and other mammals. The invention provides oligonucleotide probes and primers, polynucleotide plasmids or vectors, peptides, proteins, and antibodies relating to genes and gene products associated with the senescence in mammalian cells. The invention thus relates to the fields of molecular biology, chemistry, gerontology, pharmacology, oncology, and screening and diagnostic technology.
2. Description of Related Disclosures
Somatic cells have a finite replicative capacity (Hayflick and Moorhead, 1961, Exp. Cell Res. 25:585-621; Hayflick, 1965, Exp. Cell Res. 37:614-636; and Hayflick, 1970, Exp. Geront. 5: 291-303). This process is a major etiological factor in aging and age-related disease (Goldstein, 1990, Science 249:1129-1133; Stanulis-Praeger, 1987, Mech. Aging Dev. 38:1-48; and Walton, 1982, Mech. Aging Dev. 19:217-244). As cells undergo replicative senescence in vitro and in vivo, cells not only lose the ability to divide in response to growth stimuli, but also exhibit significant deleterious changes in the pattern of gene expression (West, 1994, Arch. Derm. 130:87-95). As an individual grows older, senescent cells make up an increasing percentage of the cells present in the tissues of the aging individual. The altered pattern of gene expression exhibited by senescent cells contributes significantly to age-related pathologies. Reversal of, or a delay in the onset of, senescence provides an effective therapy for diseases in which replicative senescence is a factor.
One fundamental cause of cellular senescence is the progressive loss of telomeric DNA in somatic cells that lack the enzyme, telomerase (Nakamura et al., Aug. 15, 1997, Science 277:955 et seq.). This arrest appears to be mediated by a DNA checkpoint by which the cell recognizes the shortened telomere as damaged DNA and causes cell cycle arrest similar to that observed in normal cells after DNA damage.
As cells progress to a senescent state, the cells exhibit an elongation of the G1 phase of the cell cycle, leading to a longer cell time of cycle transit. As the progression from a mitotically active to a senescent state continues, cells fail to respond to mitotic signals and remain in G1. This inability of senescent cells to enter the cell cycle represents a significant difference between young and old cells. Unlike old cells, young cells become quiescent entering G0 but can be subsequently induced to reenter the cell cycle and divide. However, senescent cells, while remaining viable and metabolically active, become refractile to entering the cell cycle.
A characteristic of replicative senescence is that changes in the pattern of gene expression can be observed as the cell progresses through its replicative lifespan. These changes are reflected in a decrease in the expression of "young-specific" genes and an increase in the expression of "old-specific" genes. Together, these young- and old-specific genes are referred to herein as "senescence-specific" genes, where a senescence-specific gene is any gene for which the product of the gene is differentially expressed between young quiescent cells and senescent cells. Not only do these changes affect the structure and function of the senescent cell, but also such changes can influence the physiology of surrounding cells and the tissue matrix by altering the extracellular environment, i.e., in a paracrine fashion through the release of different proteins or through changes in cell-cell interactions. Several senescence-specific genes have been described (Linskens et al., PCT No. WO 96/13610, published May 9, 1996 and incorporated herein by reference).
Changes in mRNA levels and cellular content of specific proteins provides evidence of a senescence-specific program of gene expression, leading to a differentiated genotypic and phenotypic state (Linskens et al., supra). Changes in steady state mRNA levels can translate into changes in protein expression and levels, as the rate and extent of mRNA translation represents an additional mechanism of controlling gene expression at the protein level. Cell structure and function also change when gene expression at the protein level changes.
Thus, as an individual grows older and the percentage of senescent cells in the aging individual's tissues increases, the resultant altered pattern of gene expression can contribute significantly to the pathophysiology of age-related changes in specific tissues (e.g., skin) and age related diseases. There is a need for therapeutic agents and treatments targetting the underlying biology of aging and age-related diseases, particularly the biology relating to the fundamental changes in gene expression that occur with cell senescence and lead to the development of age-related disease. The present invention helps meet that need by providing new methods and agents for discriminating between young and senescent cells, for identifying agents that modulate senescent gene expression, and for treating diseases induced or exacerbated by cellular senescence.